Balanced/unbalanced transformers (baluns) are a key component of double-balanced mixer and push-pull amplifier designs in wireless systems. They provide balanced outputs from an unbalanced input. Balanced output for wireless applications requires half the input signal amplitude at each of two output terminals, 180 degrees out of phase with each other. In principle, conventional coil transformer designs using wire wound coils, can produce this result. However, conventional wire wound devices have an upper frequency limit of several hundred megahertz due to magnetic flux leakage and capacitive coupling between the windings. Current wireless applications require very high frequency operation at low power. Active balun designs provide high frequency but operate with high DC power consumption. Passive baluns are therefore preferred. Of the known passive balun designs, Marchand type devices have become the device of choice for wireless applications. They provide excellent balance and can be made in small, easily integrated, geometries. A preferred Marchand balun, from the standpoint of miniaturization, is the spiral coil type. A version of the spiral Marchand balun has been reported by T. Gokdemir et al., IEEE MTT-S Int'l Microwave Symp. Dig., pp. 401-404. They implemented the spiral balun using GaAs MMIC technology and two side-by-side spiral microstrip lines.
Chen et al. have also reported monolithic passive balun designs using meandered line configurations. See Chen et al., “Broadband Monolithic Passive Baluns and Monolithic Double-Balanced Mixer”, IEEE Trans. Microwave Theory Tech., Vol. 39, No. 12, pp. 1980-1991. These designs have “rectangular spiral” configurations with air bridges to access the strip lines.
A more compact balun design is described in U.S. Pat. No. 6,097,273, issued Aug. 1, 2000. This design uses a thin film stack of spiral loops with multiple loops on different planes in an offset but overlying relationship.
U.S. Pat. No. 6,396,362 describes balun transformers on a conventional integrated circuit substrate pointing out the problems associated with implementing multi-level balun devices using integrated circuit technology.
There continues to be a need for high frequency, low power, baluns that are compact and can be easily and efficiently integrated.